In the field of event technology, loudspeaker systems are known that are tailored to the specific requirements associated with supplying even large audience areas with sufficient acoustic energy, also known as Public Address systems, PA systems for short. These take the form of loudspeaker units, typically configured as multi-path systems and equipped with high efficiency sound transducers that are adapted to the respective frequency range. Equipment configurations that are used in this field include single speaker units or speaker units that have been combined to create one large speaker unit, called line-arrays. If the line arrays are dimensioned appropriately, it is possible to generate the sound pressure requested by the event organiser even in regions of the audience area distant from the loudspeakers.
For a single speaker unit, the radiation corresponds essentially to the non-directional radiation of a point sound source. Accordingly, the sound pressure is halved, reduced by about 6 dB, with each doubling of the distance from the sound source. For this reason, when dealing with large audience areas increasing use is being made of line arrays. In the fundamental range, line arrays produce cylindrical waves. The surface area of a cylinder only increases linearly with its radius, not quadratically like that of a sphere. Accordingly, the sound pressure diminishes correspondingly more slowly, more specifically by about 3 dB for each doubling of the distance. The sound pressure is not reduced by half until four times the distance. Moreover, the line array offers the extra advantage that with speaker units arranged one on top of the other the sound can be directed in the elevation plane. This reduces the ambient noise ratio, which at open-air events is broadcast throughout the audience area and into the surrounding area, such as residential districts. However, the bass range is emitted non-directionally by separately mounted subwoofers.
The best way to keep the radiated wave fronts limited to the audience area is if they are aligned in both the azimuthal and elevation planes. It is possible to align wave fronts even more precisely using the wave field synthesis method described by A. J. Berkhout in 1988 [3].
If this method as described in [4] is applied in a two-dimensional array of sound transducers, the “Acoustic Curtain” is created. From a single mono signal that is convoluted into a pulse response, or from the corresponding calculations of sound propagation time and level from the distance between a virtual sound source and the respective transducer in a model-based approach, the signals can be obtained that would be picked up by a loudspeaker from a microphone arranged directly behind and in a dividing wall from a real sound source at the position of the virtual sound source. The wave front of a real sound source is reconstructed as if through a “curtain”.
Such an “acoustic curtain” according to the model-based approach is known. Characteristic of this method is that each virtual sound source behind this arrangement, is physically reconstructed from a plurality of individual transducers according to Huygens' principle. The curvature of the wave front resembles that of a wave front that might be emitted by a real sound source at the position of the virtual sound source. Thus, the virtual sound source does not change its output point with the position of the listener, like the phantom sound sources in the psycho-acoustically based methods.
Accordingly, apart from diffraction effects due to the finite area of the sound transducer arrangement, it can also only be heard in the range in which the virtual sound source is located within the arrangement of sound transducers from the point of view of the listener.
In the field of event technology, it is in principle possible to use this circumstance as a distinct advantage over the public address (PA) systems described above. The radiation direction emission of the signal and the aperture angle of the wave front relative to the sound transducer arrangement can be defined very easily with the position of the virtual sound source. Thus, radiation in the azimuthal and elevation planes can be directed straight at the audience area. For this, a virtual sound source is positioned at a considerable distance behind the sound transducer arrangement. The curvature of the wave front then corresponds to the spherical sector in the region of the sound transducer arrangement. An infinitely distant virtual sound source produces a parallel wave front, the sound level of which in theory is not diminished by distance from the sound transducer.
In this context, the sound transducer arrangement functions in the bass range like a piston-type transducer. Even large wavelengths of the signal can still be directed toward the audience area depending on the overall magnitude of the sound transducer arrangement. Thus, the alignment of the wave fronts which is controllable in the azimuthal and elevation plane can significantly reduce the interference noise ratio that travels beyond the event site at open air events.
In addition, all wave fronts arise from a common starting point. Consequently, the clearly perceptible phase problems that inevitably accompany a spatially separate setup of different speaker units do not occur. The large piston transducer, which is created from the individual transducers in the bass range, can work as fast as any individual speaker. The partial oscillations that are otherwise unavoidable on a large speaker membrane do not arise.
In practical applications, this electronically controllable sound broadcast has other advantages over fixed directional systems. Because of the more accurate directional control of the wave fronts, the proportion of the direct sound that reaches the listener is significantly increased compared to the sound components that are reflected back diffusely from the reflection surfaces. This increases the degree of clarity of the transmission and improves the intelligibility of speech. Particularly if unfavourable acoustic conditions prevail at the performance site, this is essential for high-quality transmission. Moreover, a radiation with a small aperture angle also solves a problem that is associated with conventional PA systems, specifically that sound pressure levels so high that they can be injurious to health are often produced in the area close to the stage when more distant audience areas are to be supplied with a sufficiently high sound pressure level.
Even so, the principle of the “acoustic curtain” with an arrangement of individual emitters based on the principle of wave field synthesis has not yet been applied commercially in the PA area. The advantages of radiation with a small aperture angle are lost if an expansive audience area is to be supplied with sound.
If a virtual sound source is positioned so that the wave front produced supplies an expansive audience area, a correspondingly high sound pressure level must be generated near to the sound transducer arrangement, which is then attenuated sharply with increasing distance. Thus, the advantage offered by such a sound transducer arrangement of being able to supply distant audience areas with almost the same sound pressure level as the area directly in front of a stage at a large event by radiating in a small aperture angle is lost.
In addition, with such a wide emission of a wave front with the sound transducer arrangement, it is also very difficult to achieve an adequate sound pressure level in the distant audience areas as well. In the long wavelength range, i.e. in the bass and midrange, the wide-area arrangement of sound transducers has the advantage of better adaptation to the characteristic resistance of air. A problem with conventional loudspeakers in this regard is that the air simply flows around the speaker unit in this range. The sound pressure generated is then propagated in all directions, only a fraction of the energy generated reaches the area where the audience is located. Individual speaker chassis must remain much smaller than the wavelength of the signal they generate in the bass range, because otherwise their membranes would become unstable. This is why they are almost completely ineffective in this range, the moving membrane encounters hardly any load resistance. Because of this mismatch, the efficiency of individual dynamic speakers is very low in the bass range.
This problem is solved with a sufficiently large, two-dimensional device of sound transducers according to the principle of wave field synthesis. In the bass range, the individual transducers work almost synchronously, adjacent speakers produce almost identical sound pressure at the same time. The air can no longer escape to the side, because the neighbouring speaker is producing the same air pressure there at the same time. The movement of the membrane now encounters the inertia of an air column that extends farther and farther in front of the sound transducer arrangement with increasing total surface as a working resistance. This significantly improves the efficiency of the radiation. The effect is similar to horn speakers in which the sound guide prevents the air column from escaping. Here too, the self-resonance of the sound transducer is shifted downward by the extra air mass in front of the membrane, and the efficiency is significantly increased.
Unfortunately, this advantage of the sound transducer arrangement becomes less marked with increasing frequency. In the upper transmission range, the diameters of individual sound transducers also come into the range of the wavelengths of the signal that is to be radiated. The problem of mismatch is lost here, even single emitters can already achieve high efficiency in this range, which is expressed in their sound pressure level (SPL) sensitivity.
In order to generate a sound pressure comparable with that of the conventional line arrays using the arrangement of single emitters according to the principle of wave field synthesis with a wide radiation angle of the wave front for the distant audience areas and at the upper end of the transmission range, such sound transducers that are capable of generating a sound output of the same order as their counterparts in the conventional applications would then have to be used in the sound transducer arrangement. Given the large number of individual emitters needed, the use of the arrangement of individual transducers in the PA area is not financially justifiable.